the rpki-client external test should use relase version not a master branch on github

the rpki-client external test should use relase version not a master branch on github

@botovq asked me not to use master branch of rpki-client. This will keep CI-run
green regardless of what happens in rpki-client active development branch.
Once new version of rpki-client will be released we will just update the
test here to fetch new msater version.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28136)